Sequential thermal cracking process

ABSTRACT

A process and system for sequentially cracking hydrocarbon. A first hydrocarbon feed is cracked at high severity low residence times and the cracked effluent is quenched by a second hydrocarbon feed which is coincidentally cracked at low severity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to thermal cracking of hydrocarbon. Morespecifically, the invention relates to a process and system forquenching cracked effluent with hydrocarbon feed and cracking thehydrocarbon quench material.

2. Description of the Prior Art

At present several processes exist for thermally cracking hydrocarbonsto produce olefins. In one process fired heaters are used to provide therequisite heat for reaction. The reactant flows through a plurality ofcoils within the fired heater, the coils being arranged in a manner thatmaximizes the rate of heat transfer to the hydrocarbon flowingtherethrough. An example of a conventional process is shown by U.S. Pat.No. 3,487,121 (Hallee). In another process heated solids are mixed withthe hydrocarbon feed in a reactor. Regardless of the process used,cracked effluent must be rapidly quenched to remove excess heat therebyterminating the cracking reactions. Both direct and indirect quench havebeen used to terminate the reaction. An example of a device to quenchcracked effluent is U.S. Pat. No. 3,910,347 (Woebcke).

It is well known that in the process of cracking hydrocarbon thereaction temperature and reaction residence time are the two primaryvariables affecting severity, conversion and selectivity. Severity isrelated to the intensity of the cracking reactions, conversion isdefined as the percent of feed transformed into a product, andselectivity is the degree to which the converted products constituteethylene. Selectivity is generally measured as a ratio of ethylene tomethane in the product gas effluent.

At low severity, selectivity is high, but because conversion is low, itis uneconomical to utilize low severity operation. Low severityoperation is conducted generally at temperatures between 1200° and 1400°F. and residence times between 200 and 1000 milliseconds. High severityand, hence, high conversion may be achieved at temperatures between1500° and 2000° F. However, selectivity is poor unless the high severityreaction can be performed at residence times below 200 milliseconds,usually between 20 and 100 milliseconds. At these very low residencetimes selectivities between 2.5 and 4.0 pounds of ethylene per pound ofmethane can be achieved, and conversion is generally over 95% by weightof feed. High severity operation, although preferred, has not beenemployed widely in the industry because of the physical limitations ofconventional fired heater reactors. One of the limitations is theinability to remove heat from the product effluent within the allowableresidence time parameter. For this reason most conventional systemsoperate at conditions of moderate severity, temperatures being between1350° and 1550° F. and residence times being between 200 and 500milliseconds. Although conversion is higher than at the low severityoperation, selectivity is low, being about two pounds of ethylene perpound of methane. But because conversion is higher, the actual yield ofethylene is greater than obtained in low severity operation.

By using short residence time at high severity conditions it is possibleto achieve selectivities of about 3:1 or greater. A number of processeshave been developed which offer high severity thermal cracking. Forexample, furnaces have been developed which contain a large number ofsmall tubes wherein the outlet of each tube is connected directly to anindividual indirect quench boiler. This process has the disadvantage ofbeing capital intensive in that the quench boiler is not common to aplurality of furnace tube outlets. Further, the high temperature wasteheat must be used to generate low temperature, high pressure steam whichis not desirable from a thermal efficiency viewpoint. Finally, high fluegas temperatures must be reduced by generation of steam in theconvection section of the heater, again limiting the flexibility of theprocess.

SUMMARY OF INVENTION

It is an object of this invention to provide a process that thermallycracks a first hydrocarbon feedstock at high severity and shortresidence times to produce improved yields of olefinic compounds,particularly ethylene, and to quench said reaction products with asecond hydrocarbon feedstock, which is coincidentally thermally crackedat low severity in the presence of the high severity reaction productsto produce additional yields of olefinic compounds at high selectivity.

It is an additional object of this invention to sequentially crack afirst hydrocarbon feedstock and a second hydrocarbon feedstock in amanner that enhances ethylene yield.

It is another object of this invention to improve the thermal efficiencyof processes wherein hydrocarbon feedstocks are thermally cracked toproduce olefins.

These and additional objects will become apparent from the descriptionof the process which follows.

A first hydrocarbon feedstock is introduced to a cracking reactor in thesection of the reactor identified as the primary reactor. The primaryreactor is operated at conditions of high severity and short residencetimes. Typical feedstocks suitable for introduction to the primaryreactor include light hydrocarbon gases, light gas oil petroleumfractions and heavy gas oil petroleum fractions. The operatingconditions of the primary reactor are at about 1600°-2000° F. and atabout 10-100 psig with a residence time for the hydrocarbons of between20 to 150 milliseconds. At these conditions the conversion ofhydrocarbons to products is over 95% by weight of the feed, whileselectivity is approximately 2.5 to 4 pounds of ethylene per pound ofmethane.

A second hydrocarbon feedstock which is preferably virgin gas oil400°-650° F. is introduced to the reactor in a zone identified as thesecondary reactor to quench the reaction products of the first feedstockby direct heat transfer. By adding the second feedstock, the temperatureof the combined streams of the cracked gas from the primary reactor andthe second feed is reduced to below 1500° F., and the high severityreactions are essentially terminated. The secondary reactor is operatedat low severity conditions, thus, the second feedstock is also thermallycracked therein. Processing conditions in the low severity reaction zoneare temperatures between 1200° to 1500° F., and pressures between 10-100psig. Reaction residence times are between about 150 to 2000milliseconds, preferably between 250 and 500 milliseconds.

DESCRIPTION OF DRAWING

A schematic diagram of the process and system is shown in the FIGURE.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The process and system in which the sequential cracking of the subjectinvention is illustrated is shown in the FIGURE in the environment of athermal regeneration cracking reactor (TRC). However, as has beenindicated, the process and system for effecting sequential cracking isalso applicable to conventional cracking systems.

In the sequential cracking process 2 shown in the FIGURE, the systemincludes a solids heater 4, a primary reactor 6, a secondary reactor 8and downstream equipment.

The downstream equipment is comprised essentially of an indirect heatexchanger 10, a fractionation tower 12 and a recycle line 14 from thefractionation tower 12 to the entry of the primary reactor 6.

The system also includes a first hydrocarbon feed line 16, a secondhydrocarbon feed-quench line 18, a transfer line 20 and an air deliveryline 22.

The first hydrocarbon feed stream is introduced into the primary reactor6 and contacted with heated solids from the solids heater 4. The firstor primary reactor 6 in which the first feed is cracked is at highseverity conditions. The hydrocarbon feed, from line 16, may be anyhydrocarbon gas or hydrocarbon liquid in the vaporized state which hasbeen used heretofore as a feed to a conventional thermal crackingprocess. Thus, the feed introduced into the primary reactor 6 may beselected from the group consisting of low molecular weight hydrocarbongases such as ethane, propane, and butane, light hydrocarbon liquidssuch as pentane, hexane, heptane and octane, low boiling point gas oilssuch as naphtha having a boiling range between 350° to 650° F., highboiling point gas oils having a boiling range between 650° to 950° F.and compatible combinations of same. These constituents may beintroduced as fresh feed or as recycle streams through the line 14 fromdownstream purification facilities e.g., fractionation tower 12.Dilution steam may also be delivered with the hydrocarbon through lines16 and 14. The use of dilution steam reduces the partial pressure,improves cracking selectivity and also lessens the tendency of highboiling aromatic components to form coke.

The preferred primary feedstock for the high severity reaction is alight hydrocarbon material selected from the group consisting of lowmolecular weight, hydrocarbon gases, light hydrocarbon liquids, lightgas oils boiling between 350° and 650° F., and combinations of same.These feedstocks offer the greatest increase improvement in selectivityat high severity and short residence times.

The hydrocarbon feed to the first reaction zone is preferably pre-heatedto a temperature of between 600° to 1200° F. before introductionthereto. The inlet pressure in the line 16 is 10 to 100 psig. The feedshould be a gas or gasified liquid. The feed increases rapidly intemperature reaching thermal equilibrium with the solids in about 5milliseconds. As mixing of the hydrocarbon with the heated solid occurs,the final temperature in the primary reactor reaches about 1600° to2000° F. At these temperatures a high severity thermal cracking reactiontakes place. The residence time maintained within the primary reactor isabout 50 milliseconds, preferably between 20 and 150 milliseconds, toensure a high conversion at high selectivity. Typically, the KSF(Kinetic Severity Function) is about 3.5 (97% conversion of n-pentane).Reaction products of this reaction are olefins, primarily ethylene withlesser amounts of propylene and butadiene, hydrogen, methane, C₄hydrocarbons, distillates such as gasoline and gas oils, heavy fueloils, coke and an acid gas. Other products may be present in lesserquantities. Feed conversion in this first reaction zone is about between95 to 100% by weight of feed, and the yield of ethylene for liquidfeedstocks is about 25 to 45% by weight of the feed, with selectivitiesof about 2.5 to 4 pounds of ethylene per pound of methane.

A second feed is introduced through the line 18 and combines with thecracked gas from the primary reactor 6 between the primary reactor 6 andthe secondary reactor 8. The combined stream comprising the secondunreacted feed, and the first reacted feed passes through the secondaryreactor 8 under low severity reaction conditions. The second feedintroduced through the line 18 is preferably virgin feed stock but mayalso be comprised of the hydrocarbons previously mentioned, includingrecycle streams containing low molecular weight hydrocarbon gases, lighthydrocarbon liquids, low boiling point, light compatible gas oils, highboiling point gas oils, and combinations of same.

Supplemental dilution steam may be added with the secondary hydrocarbonstream entering through stream 18. However, in most instances the amountof steam initially delivered to the primary reactor 16 will besufficient to achieve the requisite partial pressure reduction in thereactors 6 and 8. It should be understood that the recycle stream 14 isillustrative, and not specific to a particular recycle constituent.

The hydrocarbon feed delivered through the line 18 is preferably virgingas oil 400°-650° F. The second feed is preheated to between 600° to1200° F., and upon entry into the secondary reactor 8 quenches thereaction products from the primary reactor to below 1500° F. It has beenfound that in general 100 pounds of hydrocarbon delivered through theline 18 will quench 60 pounds of effluent from the primary reactor 6. Atthis temperature level, the cracking reactions of the first feed areessentially terminated. However, coincident with the quenching of theeffluent from the primary reactor, the secondary feed entering throughline 18 is thermally cracked at this temperature (1500° to 1200° F.) andpressures of 10 to 100 psig at low severity by providing a residencetime in the secondary reactor between 150 and 2000 milliseconds,preferably between 250 to 500 milliseconds. Typically, the KSF crackingseverity in the secondary reactor is about 0.5 at 300 to 400milliseconds.

The inlet pressure of the second feed in line 18 is between 10 and 100psig, as is the pressure of the first feed. Reaction products from thelow severity reaction zone comprise ethylene with lesser amounts ofpropylene and butadiene, hydrogen, methane, C₄ hydrocarbons, petroleumdistillates and gas oils, heavy fuel oils, coke and an acid gas. Minoramounts of other products may also be produced. Feed conversion in thissecond reaction zone is about 30 to 80% by weight of feed, and the yieldof ethylene is about 8 to 20% by weight of feed, with selectivities of2.5 to 4.0 pounds of ethylene per pound of methane.

Although the products from the high severity reaction are combined withthe second feed, and pass through the second reaction zone, the lowseverity conditions in the second reaction zone are insufficient toappreciably alter the product distribution of the primary products fromthe high severity reaction zone. Some chemical changes will occur,however these reaction products are substantially stabilized by thedirect quench provided by the second feed.

The virgin gas oils normally contain aromatic molecules with paraffinichydrocarbon side chains. For some gas oils the number of carbon atomsassociated with such paraffinic side chains will be a large fraction ofthe total number of carbon atoms in the molecule, or the gas oil willhave a low "aromaticity".

In the secondary reactor, these molecules will undergodealkylation-splitting of the paraffin molecules, leaving a reactiveresidual methyl aromatic, which will tend to react to form high boilers.The paraffins in the boiling range 400° to 650° F. are separated fromthe higher boiling aromatics in column 12 and constitute the preferredrecycle to the primary reactor.

Other recycle feed stocks can include propylene, butadiene, butenes andthe C₅ -400° F. pyrolysis gasoline.

The total effluent leaves the secondary reactor and is passed throughthe indirect quench means 10 to generate steam for use within andoutside the system. The effluent is then sent to downstream separationfacilities 12 via line 24.

The purification facilities 12 employ conventional separation methodsused currently in thermal cracking processes. The FIGURE illustratesschematically the products obtained. Hydrogen and methane are takenoverhead through the line 36. C₄ and lighter olefins, C₅ -400° F. and400°-650° F. fractions are removed from the fractionator 12 throughlines 26, 28 and 30 respectively. Other light paraffinic gases of ethaneand propane are recycled through the line 14 to the high severityprimary reactor. The product taken through line 28 consists of liquidhydrocarbons boiling between C₅ and 400° F., and is preferably exportedalthough such material may be recycled to the primary reactor 6 ifdesired. The light gas oil boiling between 400° to 650° F. is thepreferred recycle feed, but may be removed through line 30. The heavygas oil which boils between 650°-950° F. is exported through stream 32,while excess residuim, boiling above 950° F. is removed from the batterylimits via stream 34. The heavy gas oil and residuim may also be used asfuel within the system.

In the preferred embodiment of the process, the second feed would be onewhich is not recommended for high severity operation. Such a feed wouldbe a gas oil boiling above 400° F. which contains a significant amountof high molecular weight aromatic components. Generally, thesecomponents have paraffinic side chains which will form olefins underproper conditions. However, even at moderate severity, the dealkylatedaromatic rings will polymerize to form coke deposits. By processing thearomatic gas oil feed at low severity, it is possible to dealkylate therings, but also to prevent subsequent polymerization and coke formation.As a consequence of the low severity, however, the yield of olefins islow, even though selectivity as previously defined is high. Hence, lowseverity reaction effluents often have significant amounts of lightparaffinic gases and paraffinic gas oils. These light gases andparaffinic gas oils are recycled preferably to the high severitysection, such compounds being the preferred feeds thereto. The aromaticcomponents of the effluent are removed from the purification facilities12 as part of the heavy gas oil product, and either recycled for use asfuel within the system, or exported for further purification or storage.

An illustration of the benefits of the process of the invention is setforth below wherein feed cracked and the resultant product obtainedunder conventional high severity cracking and quenching conditions iscompared with the same feed sequentially cracked in accordance with thisinvention.

    ______________________________________                                        FEED           400-650° F. Gas Oil                                     PYROLYSIS MODE Once Thru Sequential TRC                                       REACTOR                   6             8                                     ______________________________________                                        FEED           Virgin    Recycle       Virgin                                 TIME, MILLISEC.                                                                              300       20            300                                    KSF            4.0       3.5           0.5                                    WT. % FEED     100       60            100                                    CRACKED PRODUCT                                                               FUEL GAS       14                 10                                          C.sub.2 H.sub.4                                                                              22                 27                                          C.sub.3 H.sub.6                                                                              8                  14                                          C.sub.4 -400° F.                                                                      11                 13                                          400-650° F.                                                                           20                 11                                          650° F.+                                                                               25                25                                                         100                100                                         ______________________________________                                    

On the sequential TRC process, the recycle feed is cracked under highseverity conditions in the primary reactor 6 and the virgin feed isintroduced through the line 18 to quench the cracked product from theprimary reactor 6. Thereafter, the virgin feed is cracked under lowseverity conditions in the secondary reactor 8.

What is claimed is:
 1. A process for cracking hydrocarbon feed toproduce olefins comprising:a. delivering hydrocarbon feed to a firstzone; b. thermally cracking the hydrocarbons in the first zone attemperatures above 1,500° F.; c. discharging the cracked effluent fromthe first zone to a second zone; d. delivering a second hydrocarbon feedto the entry of the second zone; and e. mixing the cracked effluent fromthe first zone and the second hydrocarbon feed in the secondzone;whereby the cracked effluent from the first zone is quenched andthe second hydrocarbon feed is cracked at low severity in the secondzone at a residence time of the hydrocarbon between 150 and 2,000milliseconds, a hydrocarbon temperature of 1,200° to 1,500° F. and apressure of 10 to 100 psig.
 2. A process as in claim 1 furthercomprising the steps of passing the composite quenched effluent from thesecond zone through the hot side of an indirect heat exchanger andpassing steam through the cold side of the indirect heat exchanger.
 3. Aprocess as in claim 1 further comprising the steps of fractionating thecracked effluent and returning a portion of the fractionated crackedeffluent to the first zone.
 4. A process as in claim 1 wherein the firstzone is operated at high severity short residence cracking conditions.5. A process as in claim 1 wherein the feedstock delivered to the firstzone is selected from the group consisting of low molecular weighthydrocarbon gases, light hydrocarbon liquids, light gas oils boilingbetween 350° and 650° F., and combinations of said hydrocarbons.
 6. Aprocess as in claim 1 wherein the feed delivered to the second zone isvirgin gas oil 400° to 650° F.
 7. A process as in claim 3 wherein thefraction returned to the first zone is light paraffinic gases of ethaneand propane.
 8. A process as in claim 1 wherein the hydrocarbondelivered to the first zone is pre-heated to a temperature between 600°and 1,200° F.
 9. A process as in claim 1 wherein the hydrocarbondelivered to the second zone is pre-heated to a temperature between 600°and 1,200° F.
 10. A process as in claim 4 wherein the reactionconditions in the first zone are a residence time of 20 to 150milliseconds, a hydrocarbon temperature of 1,600° to 2,000° F. and apressure of 10 to 100 psia.
 11. A process as in claim 10 wherein thekinetic severity function in the first zone is about 3.5.
 12. A processas in claim 1 wherein 100 pounds of hydrocarbon are delivered to thesecond reaction zone as quench for every 60 pounds of effluent from theprimary zone.
 13. A process as in claim 1 wherein the kinetic severityfactor in the second zone is about 0.5 at about 300 to 400 milliseconds.